Motor control apparatus and method

ABSTRACT

An apparatus for controlling an AC power supply for an electric motor, said AC power supply being derived from a DC voltage. The apparatus comprises a comparer configured to provide a comparison of a modulation index of a motor control signal with a reference value. This current data provider is configured to provide current data based on a torque demand signal; a speed signal indicating the speed of rotation of the AC motor; and an indication of the DC voltage modified on the comparison for control of the motor control signal which is based on the motor current data.

The present disclosure relates to apparatus and methods for the controlof electric motors, and more particularly for methods and apparatus forthe control of permanent magnet motors, still more particularly to fieldweakening strategies for permanent magnet AC motors.

Permanent magnet motors offer a number of advantages, and their use invarious applications is increasing. In the control of permanent magnetmotors field weakening phenomena may be exploited to control motorperformance. In particular, field weakening schemes enable the operatinglimits of permanent magnet motors to be expanded by modifying thecurrent vector control in view of the modulation index.

Particular examples and features of the disclosure are set out in theappended claims.

Embodiments of the disclosure will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of an apparatus comprising an electricmotor;

FIG. 2 shows a DQ data provider for use in the apparatus of FIG. 1;

FIG. 3 shows a flow diagram indicating a method of controlling a motor;and

FIG. 4 shows a further example of the apparatus of FIG. 2.

In overview, the apparatus of FIG. 1 is arranged to control a salient ornon-salient pole permanent magnet AC, PMAC, motor 4, based on torquedemand, and the torque-speed characteristics of the motor 4.

As will be appreciated by the skilled addressee in the context of thepresent disclosure, the current in the windings of such a motor may beconsidered in a reference frame which rotates with the rotor of themotor. In this reference frame, the current in the motor windings has adirect phase component, in phase with the rotor, and a quadrature phasecomponent π/4 out of phase with the rotor. These current components arereferred to as D and Q currents respectively, I_(D), I_(Q).

The inventors in the present case have appreciated that pole PMAC motorscannot be controlled using a traditional strategy of holding the directphase current, I_(D), at zero and applying quadrature phase current,I_(Q), in proportion to the torque demand because both current phases,I_(D) and I_(Q) produce torque. To address this I_(D) and I_(Q) data maybe indexed by motor speed and torque demand to enable the motor to becontrolled to produce a required torque according to its characteristicsat a given speed. In addition, for field weakening, the example of FIG.1 also indexes the I_(D) and I_(Q) data based on DC link voltage.

In an embodiment, to control the modulation index the voltage used toindex the I_(D) and I_(Q) data is reduced in proportion to the amount bywhich the modulation index exceeds a threshold. Typically the I_(D) andI_(Q) data are defined for a limited range of DC link voltages, and fora limited range of speeds and torque demands.

Once the modulation index adjustment of the DC link voltage has reachedthe limit of the defined voltage range, a first additional stage offield weakening adjustment may be applied to reduce the torque demandused to index the I_(D) and I_(Q) data.

Once the limit of this first additional adjustment stage has beenreached (e.g. the minimum torque demand for which I_(D) and I_(Q) datavalues are defined), the I_(D) current itself may be reduced in a secondadditional stage of field weakening.

FIG. 1 illustrates one system for putting this disclosure into effect.

In FIG. 1 an AC power provider 2 is coupled to an AC motor 4, and a DCpower provider 6. The AC power provider is also coupled to a controller8. The controller 8 is also coupled to a DQ data provider 12. A speeddeterminer 10 is coupled to the motor 4 and to the DQ data provider 12.

The DC power provider 6 is operable to provide a DC voltage to the ACpower provider 2. The DC power 6 provider may also be coupled to providepower for other elements of the apparatus of FIG. 1, but in theinterests of clarity the individual power couplings are not shown in thediagram.

The AC power provider 2 is operable to derive a multi-phase AC powersupply for the motor 4 from the DC voltage, and to control the timingand power of the AC power supply based on a motor control signalreceived from the controller 8.

The motor 4 comprises a pole permanent magnet AC motor (in this examplea salient pole permanent magnet AC motor), arranged to receive themulti-phase AC power supply from the AC power provider. The motor 4comprises windings arranged to provide a rotating magnetic field inresponse to the multi-phase AC power supply.

The controller 8 is configured to receive an indication of requireddirect phase, and quadrature phase currents and to provide to the ACpower provider 2 a control signal for controlling the AC power provider2 to provide a three phase AC power supply for the AC motor 4. Themodulation index of this control signal may be defined by the ratio ofthe control signal's amplitude to the available dynamic range of thecontroller 8, e.g. the control signal amplitude divided by the maximumavailable control signal voltage. In a pulse width modulation, PWM,controller the modulation index may be defined by the ratio of thecontrol signal amplitude to the timing or clock signal used to derivethe PWM output signal.

The speed determiner 10 is arranged to determine the speed of rotationof the motor 4 and to provide a speed signal to the DQ data provider 12.

The DQ data provider 12 is configured to receive a torque demand input,a speed signal from the speed determiner 10, and an indication of the DCvoltage from the DC power provider 6. The DQ data provider is alsoconfigured to receive an indication of the modulation index of the motorcontrol signal provided by the controller 8 to the AC power provider 2.The DQ data provider is further configured to select direct phase andquadrature phase current values selected for the motor 4 based on themodulation index, the DC voltage, the motor speed and the torque demandand to provide to these current commands to the controller 8 based onthe selected values. The DQ data provider is configured to perform thisselection according to characteristics of the motor 4 defined in astored look-up table relating the speed, torque demand and voltage todirect phase and quadrature phase current values.

In operation, the speed determiner determines the speed of the motor 4and passes a speed signal to the DQ data provider 12. The DQ dataprovider 6 also receives a torque demand signal, an indication of thelink DC voltage of the AC power provider 2, and the modulation index ofthe control signal provided by the controller 8 to the AC power provider2.

Based on the received signals the DQ data provider 6 selects phasecurrent I_(D), and quadrature phase current I_(Q) commands. The selectedI_(Q) and I_(D) commands are provided to the controller 8 whichgenerates a PWM control signal for controlling the AC power provider 2.The AC power provider 2 provides a multi-phase AC power supply to themotor 4 in accordance with the selected I_(D), I_(Q) data.

The operation of the DQ data provider 12 will be described in greaterdetail with reference to FIGS. 2, 3 and 4 below. However, in summary,the DQ data provider 12 is configured to apply a three stage fieldweakening scheme. In the first stage, the received modulation index iscompared with a threshold, and the DC voltage used to select theI_(D)-I_(Q) data is reduced according to the amount by which themodulation index exceeds that threshold. This reduces the modulationindex by reducing the commanded I_(D) and/or I_(Q) current.

If the modulation index continues to increase, the DC voltage willultimately be reduced to the minimum DC voltage for which theI_(D)-I_(Q) data is defined. In that eventuality, the DQ data provider12 of FIG. 1 is configured to apply an additional stage of fieldweakening adjustment by reducing the torque demand used to select theI_(D) and I_(Q) data, thereby in turn reducing the modulation index.

Nevertheless, if the modulation index continues to increase stillfurther, the reduced torque demand may reach the minimum value for whichI_(D) and I_(Q) commands are defined, and in that eventuality, the DQdata provider 12 of FIG. 1 is configured to reduce the I_(D) currentcommand provided to the controller 8.

One example of an apparatus to provide the functionality of the DQ dataprovider 12 of FIG. 1 is described with reference to FIG. 2.

The apparatus of FIG. 2 comprises a comparer 20 coupled to receive amodulation index signal and coupled to a first combiner 28. The firstcombiner 28 is coupled to receive an indication of the DC voltage, andto receive the output of the comparer 20. The output of the firstcombiner 28 is coupled to a torque demand modifier 22.

The torque demand modifier 22 is coupled to provide a first output to acurrent data provider 26, and a second output to a second combiner 30.The second combiner 30 is also coupled to receive a torque demandsignal, and to provide an output to a current data modifier 24. Thecurrent data modifier is coupled to provide a first output to thecurrent data provider 26, and a second output to a third combiner 32.The current data provider 26 is coupled to provide a quadrature currentoutput, I_(Q), and to provide an in phase current output I_(D) to thethird combiner 32.

The current data provider 26 comprises a plurality of storedassociations, each of which associates one of a plurality of currentdata elements with a respective: motor speed, torque demand, and DCvoltage. Thus the current data provider 26 is operable to providecurrent data by selecting amongst stored data elements based on theseassociations. The associations define current data for a selected rangeof DC voltage values, a selected range of motor speed values, and aselected range of torque demand values. These ranges are referred toherein as “the voltage range”, “the speed range”, and “the torque range”respectively.

The comparer 20 is operable to compare the received modulation indexwith a reference, to determine the difference between the modulationindex and this reference. This enables the comparer to produce an outputbased on this difference. The comparer 20 is configured to clip thisoutput to ensure that it is not negative, and does not exceed a selectedmaximum output. The combiner 28 is arranged to subtract the clippedoutput of the comparer 20 from the DC voltage input. The output from thefirst combiner 28 is coupled to the torque demand modifier 22.

The torque demand modifier is configured to provide two outputs. Thefirst output is the output from the first combiner 28 (e.g. modified DCvoltage input) clipped to within the voltage range of the current dataprovider 26, e.g. DC voltage values less than the minimum value in thevoltage range are mapped to that minimum, and DC voltage values whichare greater than the maximum value in the voltage range are mapped tothat maximum value. This first output is coupled to the current dataprovider 26 to enable the selection of current data based on this firstoutput, e.g. the first output is the DC voltage input modified to takeaccount of an excess in the modulation index, so that by reducing the DCvoltage the modulation index can be controlled.

The second output from the torque demand modifier is based on thedifference between the input and the first output, e.g. the clippedvalue is subtracted from the input value to provide the second output.Thus, in the event that the modified DC voltage lies outside the voltagerange of the current data modifier 26, the second output from the torquedemand modifier 22 is based on the difference between the boundary ofthe voltage range and the modified DC voltage. By contrast, in the eventthat the input to the torque demand modifier 22 is within the voltagerange, the second output from the torque demand modifier is zero(because no clipping takes place). Thus, in this embodiment, the voltageadjustment is applied first, and only once the voltage adjustment hasreached its limit is the torque demand adjusted.

The second output of the torque demand modifier 22 is combined with thetorque demand signal by the combiner 30 to provide a modified torquedemand. This modified torque demand is coupled to the current datamodifier 24.

The current data modifier 24 is configured to provide two outputs. Thefirst output from the current data modifier 24 is the modified torquedemand, clipped to within the torque range of the current data provider26. This output is coupled to the torque demand input of the currentdata provider 26. The second output from the current data modifier isbased on the difference between the modified torque demand signal andthe first output. Thus, in the event that the modified torque demandsignal is within the torque range, the second output is zero, but in theevent that the modified torque demand lies outside the torque range, thesecond output is based on the difference between a boundary of thetorque range and the modified torque demand signal.

The second output from the current data modifier is coupled to the thirdcombiner 32 which is arranged to combine the second output with theselected in phase current data provided by the current data provider 26.Accordingly, in the event that the modified torque demand is in thetorque range, the in phase current data is not adjusted, but in theevent that the torque demand is modified out of the torque range, the inphase current data, I_(D) is modified to produce the current command.Thus, in this embodiment, the DC voltage adjustment is applied first,and once the DC voltage adjustment has reached its limit, the torquedemand adjusted, and once the torque adjustment has reached its limit,the in-phase current data, I_(D) adjusted.

The example of FIG. 2 has been set out assuming particular signalpolarities. However, as will be appreciated in the context of thepresent disclosure, the actual signal polarities may be varied if, forexample, signals are combined by being added rather than beingsubtracted or vice versa. The example of FIG. 2 illustrates certainfunctionality of the apparatus as functional elements or blocks.However, this is merely illustrative and equivalent functionality may beprovided by different subdivisions of function. For example eachillustrated functional unit may be further subdivided, or thefunctionality of two or more of the illustrated elements may beintegrated. In some examples a single processor may provide some or allof the functionality illustrated in FIG. 2. Such a processor may beprovided by a suitably programmed general purpose processor, or by anapplication specific integrated circuit, ASIC, or by a fieldprogrammable gate array, FPGA. Examples of the disclosure providecomputer program products in the form of software and/or middleware,and/or firmware, comprising program instructions operable to program aprocessor to provide the apparatus of FIG. 2.

FIG. 3 illustrates a method of controlling an electric motor based onthe DC voltage, the motor speed, and the torque demand.

The modulation index of a motor control signal is compared 100 with athreshold. In the event that the modulation index is less than thethreshold, the DC voltage, speed and torque values are passed 102 foruse in a selection 104 of in phase and quadrature phase currents for themotor. In the event that the modulation index is greater than thethreshold the DC voltage is modified 106 based on the overshoot of themodulation index.

The modified DC voltage is compared 108 with a minimum DC voltage, andin the event that the modified DC voltage is not less than the minimum,the modified DC voltage, speed and torque values are passed 110 for usein a selection 112 of in phase and quadrature phase currents for themotor. In the event that the DC voltage is less than the minimum DCvoltage, the torque demand is modified 114 based on the DC voltageundershoot.

The modified torque is compared 116 with a minimum torque, and in theevent that the modified torque is not less than the minimum torque, theminimum DC voltage, the modified torque, and the motor speed are passed118 for use in a selection 120 of in phase and quadrature phase currentsfor the motor. In the event that the modified torque is less than theminimum torque, the minimum DC voltage, the minimum torque, and themotor speed are passed 122 for use in a selection 124 of in phase andquadrature phase currents for the motor. The selected in-phase currentis then modified 126 based on the torque demand undershoot, e.g. theamount by which the modified torque demand is less than the minimumtorque demand.

The method of FIG. 3 is merely illustrative. Although the method of FIG.3 comprises a number of stages examples of the disclosure need notinclude all of these stages, and some examples with fewer stages aredefined in the appended claims. The selections 104, 112, 120 may beperformed by any selector such as a look-up table, or by an analytic ornumerical computer program. The minimum values referred to withreference to FIG. 3 may be defined by the working ranges of theseselectors. Embodiments of the disclosure provide computer programproducts operable to program a processor to perform the method of FIG.3.

FIG. 4 shows a further example of a DQ data provider for use in anapparatus such as that shown in FIG. 1, or for performing a method suchas that described with reference to FIG. 3. In FIG. 4, for clarity allunits are described in a per unit system.

In FIG. 4, a modulation index input is coupled to a subtractor 50arranged to subtract a reference value from the modulation index toprovide an error signal. The error signal output from the subtractor 50is coupled to a proportional-integral, PI, controller 54 arranged toprovide an output based on the error signal scaled by a selected gain,and the sum of the error signal over time. The output from the PIcontroller is coupled to a first saturation element 56. The saturationelement 56 is configured to clip the output of the PI controller 54 toremain between zero (0) and one (1).

The output of the first saturation element 56 is coupled to a subtractor58 which is arranged to subtract the clipped PI controller output fromthe DC voltage to provide a modified DC voltage value. The output of thesubtractor 58, the modified DC voltage, is coupled to a secondsaturation element 60 which clips the modified DC voltage to within aselected range of voltages. The output of the saturation element 60 iscoupled to an input of a 3D look up table 62. The output of thesaturation element 60 is also coupled to a second subtractor 64 which isarranged to subtract the clipped modified voltage from the modifiedvoltage. Thus, in the event that the modified DC voltage is within theselected voltage range defined by the second saturation element 60, theoutput of the subtractor 64 is zero. The output of the subtractor 64 iscoupled to a third saturation element 66 configured to clip the outputof the subtractor 64 so that it remains within a specified range, forexample the range of zero (0) to minus 1 (−1).

The output of the third saturation element 66 is coupled to a summingelement 68 arranged to add the (negative) output from the saturationelement 66 from the torque demand to provide a modified torque demand.The modified torque demand is coupled to a fourth saturation element 70arranged to clip the modified torque demand so that it is limited to aselected range of torque demands. The output of the saturation elementis coupled to the torque demand input of the 3D look up table 62.

The output of the fourth saturation element 70 is also coupled to asubtractor 72 arranged to subtract the clipped modified torque demandfrom the modified torque demand value. Thus in the event that themodified torque demand is within the selected range defined by thesaturation element 70, the output from the subtractor 72 is zero.

The output from the subtractor 72 is coupled to a fifth saturationelement 74 arranged to limit the output from the subtractor to within aselected range. The output from the fifth saturation element 74 iscoupled to a second PI controller 76 arranged to determine a currentadjustment value based on the undershoot of the modified torque value(e.g. the difference between the modified torque value and the minimumtorque value in the range of the look-up table 62. The currentadjustment value from the second PI controller 76 is coupled to asumming element 78. The summing element 78 is also coupled to thein-phase current output of the 3D look up table 62. The 3D look-up table62 is configured to provide current data I_(D), I_(Q) in response toreceived torque demand, speed, and DC voltage values.

In operation, in the event that the modulation index is less than thethreshold level provided by the reference 52, the 3D look up tableprovides the stored I_(D), and I_(Q) values for the DC voltage, torquedemand and motor speed. In the event that the modulation index isgreater than that threshold, the field weakening scheme is applied byadjusting the DC voltage used to index into the look up table 62. Thisin turn reduces the modulation index.

If the modulation index continues to increase, once the range of DCvoltage adjustment has been exhausted the torque demand is adjusted.Then, once the range of torque demand adjustment has been exhausted thein-phase current selected from the look up table 78 is reduced directlyby applying a negative adjustment to the in-phase current data selectedby the look-up table 62.

The apparatus and methods described with reference to FIGS. 1 to 4 aredescribed as including a series of adjustments in which first voltage,then torque demand, then in phase current, I_(D) are reduced. However insome examples, these stages may be applied in a different sequence,and/or only some of these stages are applied. For example the torquedemand adjustment and/or the current adjustment stages may not be used.In some examples the adjustments may be applied in a different sequence,and in some cases the voltage adjustment need not be applied, e.g justthe torque adjustment and/or current could be applied.

For example, it will be understood that the DC voltage used to selectthe current commands need not be varied, and the disclosure provides anapparatus for controlling an AC power supply for an electric motor, saidAC power supply being derived from a DC voltage, the apparatuscomprising: a comparer configured to provide a comparison of amodulation index of a motor control signal with a reference value; and acurrent data provider, configured to provide current data based on: atorque demand signal, modified based on the comparison; a speed signalindicating the speed of rotation of the AC motor; and an indication ofthe DC voltage; for control of the motor control signal based on themotor current data. In an embodiment this apparatus further comprises acurrent data modifier configured to modify the provided current databased on the difference between the modified torque demand signal and aboundary of a selected torque range in the event that the torque demandsignal is outside the torque selected range.

Similarly, there is also provided an apparatus for controlling an ACpower supply for an electric motor, said AC power supply being derivedfrom a DC voltage, the apparatus comprising: a processor configured to:compare a modulation index of a motor control signal with a referencevalue; provide current data based on: a torque demand signal modifiedbased on the comparison; a speed signal indicating the speed of rotationof the AC motor; and an indication of the DC voltage; to enable controlof the motor control signal based on the motor current data. Inaddition, in one example there is provided a method of controlling apower supply for an AC motor, said AC power supply derived from a DCvoltage, the method comprising: selecting data from stored motor currentdata based on: a selection signal based on a torque demand signal and acomparison between a reference value and a modulation index of a motorcontrol signal; the speed of rotation of the AC motor; and the DCvoltage; and controlling the motor control signal in accordance with theselected motor current data.

As another example, it will be understood that the DC voltage used toselect the current commands need not be varied, and the torque demandalso need not be varied, according the disclosure provides an apparatusfor controlling an AC power supply for an electric motor, said AC powersupply being derived from a DC voltage, the apparatus comprising: acomparer configured to provide a comparison of a modulation index of amotor control signal with a reference value; and a current dataprovider, configured to provide current data based on: a torque demandsignal; a speed signal indicating the speed of rotation of the AC motor;and an indication of the DC voltage; for control of the motor controlsignal based on the motor current data, and a current data modifierconfigured to modify the provided current data based on the comparison.

Similarly, there is also provided an apparatus for controlling an ACpower supply for an electric motor, said AC power supply being derivedfrom a DC voltage, the apparatus comprising: a processor configured to:compare a modulation index of a motor control signal with a referencevalue; provide current data based on: a torque demand signal; a speedsignal indicating the speed of rotation of the AC motor; and themodified indication of the DC voltage; for control of the motor controlsignal based on the motor current data, and to modify the providedcurrent data based on the comparison. In addition, in another examplethere is provided a method of controlling a power supply for an ACmotor, said AC power supply derived from a DC voltage, the methodcomprising: selecting current data from stored motor current data basedon: a torque demand signal; the speed of rotation of the AC motor; andthe DC voltage and a comparison between a reference value and amodulation index of a motor control signal; and modifying the selectedcurrent data based on a comparison between a reference value and amodulation index of a motor control signal, and controlling the motorcontrol signal in accordance with the selected modified motor currentdata.

In an aspect there is provided an apparatus for controlling an AC powersupply for an electric motor, said AC power supply being derived from aDC voltage, the apparatus comprising: a comparer configured to provide acomparison of a modulation index of a motor control signal with areference value; and a current data provider, configured to providecurrent data based on: a torque demand signal; a speed signal indicatingthe speed of rotation of the AC motor; and an indication of the DCvoltage, modified based on the comparison; for control of the motorcontrol signal based on the motor current data.

In an aspect there is provided an apparatus for controlling an AC powersupply for an electric motor, said AC power supply being derived from aDC voltage, the apparatus comprising: a processor configured to: comparea modulation index of a motor control signal with a reference value;modify an indication of the DC voltage based on the comparison; providecurrent data based on: a torque demand signal; a speed signal indicatingthe speed of rotation of the AC motor; and the modified indication ofthe DC voltage; for control of the motor control signal based on themotor current data.

In an aspect there is provided a method of controlling a power supplyfor an AC motor, said AC power supply derived from a DC voltage, themethod comprising: selecting data from stored motor current data basedon: a torque demand signal; a speed signal indicating the speed ofrotation of the AC motor; and a selection signal based on the DC voltageand a comparison between a reference value and a modulation index of amotor control signal; and controlling the motor control signal inaccordance with the selected motor current data.

Each of the foregoing examples and variations are intended to becombined with other features of the examples of the disclosure discussedelsewhere herein, and in particular to be combined with the features setout in each of the appended claims.

The invention claimed is:
 1. An apparatus for controlling an AC powersupply for an electric motor, said AC power supply being derived from aDC voltage, the apparatus comprising: a comparer configured to provide acomparison of a modulation index of a motor control signal with areference value; and a current data provider, configured to providecurrent data based on: a torque demand signal; a speed signal indicatingthe speed of rotation of the AC motor; an indication of the DC voltage,modified based on the comparison; for control of the motor controlsignal based on the motor current data; a torque demand modifierconfigured to modify the torque demand signal based on the modifiedindication of the DC voltage in the event that the modified indicationof the DC voltage is outside a first selected range, and in which thecurrent data provider is configured to provide motor current data basedon the modified torque demand signal; and a current data modifierconfigured to modify the provided current data based on the differencebetween the modified torque demand signal and a boundary of a secondselected range in the event that the torque demand signal is outside thesecond selected range.
 2. The apparatus of claim 1 comprising a datastore comprising a plurality of associations arranged to associate eachof a plurality of current data elements with a respective: motor speed,torque demand, and DC voltage; wherein the data provider is configuredto provide the current data by selecting amongst the stored dataelements based on the associations.
 3. The apparatus of claim 1 in whichthe current data comprises a direct phase current information, andquadrature phase current information.
 4. The apparatus of claim 3comprising a current data modifier configured to modify the providedcurrent data based on the difference between the modified torque demandsignal and a boundary of a second selected range in the event that thetorque demand signal is outside the second selected range in which theprovided current data comprises an in phase current value, ID and aquadrature phase current value, IQ, and in which modifying the currentdata comprises applying a negative adjustment to the in phase currentvalue ID.
 5. The apparatus of claim 1 in which the modulation index ofthe control signal comprises the ratio of the motor control signalamplitude to one of: (a) a maximum amplitude value of a controller thatprovides the motor control signal; or (b) the amplitude of a timingsignal used to determine a PWM signal from the motor control signal. 6.The apparatus of claim 1 further comprising a motor controller adaptedto provide the motor control signal based on the current data.
 7. Theapparatus of claim 6 wherein the motor controller is configured tocontrol an inverter adapted to derive said AC power supply from said DCpower supply.
 8. An apparatus for controlling an AC power supply for anelectric motor, said AC power supply being derived from a DC voltage,the apparatus comprising: a processor configured to: compare amodulation index of a motor control signal with a reference value;modify an indication of the DC voltage based on the comparison; providecurrent data based on: a torque demand signal; a speed signal indicatingthe speed of rotation of the AC motor; the modified indication of the DCvoltage; modify the torque demand signal based on the modifiedindication of the DC voltage in the event that the modified indicationof the DC voltage is outside a first selected range, and to providemotor current data based on the modified torque demand signal modify theprovided current data based on the difference between the modifiedtorque demand signal and a boundary of a second selected range in theevent that the torque demand signal is outside the second selectedrange; for control of the motor control signal based on the motorcurrent data.
 9. The apparatus of claim 8 comprising a memory storing aplurality of associations between each of a plurality of current dataelements with a respective: motor speed, torque demand, and DC voltage;wherein the processor is coupled to the memory and configured to providethe current data by selecting amongst the stored data elements based onthe associations.
 10. A method of controlling an AC power supply for anAC motor, said AC power supply derived from a DC voltage, the methodcomprising: selecting data from stored motor current data based on: atorque demand signal; a speed signal indicating the speed of rotation ofthe AC motor; and a selection signal based on the DC voltage and acomparison between a reference value and a modulation index of a motorcontrol signal; modifying the torque demand signal based on theselection signal in the event that the selection signal is outside afirst selected range, and selecting the data based on the modifiedtorque demand signal; modifying the selected data based on thedifference between the torque demand and a boundary of a second selectedrange in the event that the modified torque demand signal is outside thesecond selected range; and controlling the motor control signal inaccordance with the selected motor current data.
 11. The method of claim10 in which the selected data comprises an in phase current value, ID,and a quadrature phase current value, IQ, and in which modifying theselected data comprises applying a negative adjustment to the in phasecurrent value ID.
 12. The method of claim 10 in which the modulationindex of the control signal comprises the ratio of the motor controlsignal amplitude to one of: (a) a maximum amplitude value of acontroller that provides the motor control signal; or (b) the amplitudeof a timing signal used to determine a PWM signal from the motor controlsignal.